Building temperature control system and method

ABSTRACT

A method and system of dynamically controlling a temperature control system that has at least one compressor and is operable in a plurality of stages. The method includes iteratively determining, for each of the plurality of stages, a plurality of intermediate air-related conditions and system operating conditions based on sensed conditions. The method also includes identifying a stage among the plurality of stages, and updating an initial supply air temperature set point with a supply air temperature set point corresponding to the identified stage.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/701,608, filed on Jul. 22, 2005, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments of the invention relate to temperature control systems and methods for buildings and other structures.

BACKGROUND

Various types of facilities, such as buildings, industrial production facilities, medical buildings, manufacturing assemblies, and laboratories, often use air handling units (“AHUs”) to control indoor temperatures. An AHU generally uses outside air, compressors, and fans to supply air at designated temperatures to different areas, zones, or rooms. In some cases, an AHU includes an economizer that reduces energy consumption by the AHU.

An AHU typically sets a temperature limit for outside air (i.e., a supply air temperature), such as, for example, about 55° F. Fluctuations of outside air temperature result in an AHU switching compressors on and off frequently, which can consume significant energy and put significant stress on the compressors.

SUMMARY

Embodiments of the invention provide energy-efficient control systems and methods that can be retrofitted in existing temperature control systems, or can be incorporated in new systems.

In one embodiment, the invention provides a method of dynamically controlling a temperature control system that includes at least one compressor and is operable in a plurality of stages. The method includes modulating the compressor based on an initial supply air temperature set point, and determining a first plurality of air-related conditions and system operating conditions. The method also includes iteratively determining a plurality of intermediate air-related conditions and system operating conditions based on the determined first plurality of conditions, for each of the plurality of stages. The method also includes comparing at least some of the intermediate conditions with a plurality of corresponding thresholds, identifying a stage among the plurality of stages based on the comparing, and updating the initial supply air temperature set point with a supply air temperature set point corresponding to the identified stage.

In another embodiment, the invention provides a controller for dynamically controlling a temperature control system. The temperature control system is operable in a plurality of stages and is further operable to cool air in a location, and includes at least one compressor, a modulator configured to modulate the compressor based on an initial supply air temperature set point, and a plurality of sensors operable to sense a plurality of air-related conditions and system operating conditions. The controller includes an iteration module, a comparator, an identifier module, and an updater module. The iteration module iteratively determines, for each of the plurality of stages, a plurality of intermediate air-related conditions and system operating conditions based on the sensed conditions. The comparator module compares at least some of the intermediate conditions with a plurality of corresponding thesholds. The identifier module identifies a stage among the plurality of stages based on at least one comparison of the comparator module. The updater module updates the initial supply air temperature set point with a supply air temperature set point corresponding to the identified stage.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an air handling unit (“AHU”).

FIG. 2 is a side view of a fan that can be implemented in the AHU of FIG. 1.

FIG. 3 is a front view of the fan of FIG. 2.

FIG. 4 is a block diagram of a controller that can be implemented in the AHU of FIG. 1.

FIG. 5 is a flow chart illustrating exemplary processes carried out in the controller of FIG. 4.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

As should also be apparent to one of ordinary skill in the art, the systems shown in the figures are models of what actual systems might be like. Many of the modules and logical structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “controller” may include or refer to both hardware and/or software. Furthermore, throughout the specification capitalized terms are used. Such terms are used to conform to common practices and to help correlate the description with the coding examples, equations, and/or drawings. However, no specific meaning is implied or should be inferred simply due to the use of capitalization. Thus, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware.

Also, as used herein, the term “refrigerant” refers to a fluid used for heating, cooling, and/or defrosting purposes, such as, for example, chlorofluorocarbons (“CFCs”), hydrocarbons, cryogens (e.g., CO₂, and N₂), etc.

Embodiments of the invention provide control systems and methods that can be retrofitted in existing temperature control systems, or can be incorporated in new systems. In one particular embodiment, a controller in a temperature control system iteratively determines intermediate conditions based on sensed and/or computed conditions, compares at least some of the intermediate conditions with corresponding thresholds, and selects an iteration that meets certain requirements. By controlling the system in accordance with the selected iteration, the controller can maximize free cooling, minimize reheat, and reduce fan power consumption in the system. In addition, embodiments herein can increase the lifespan of compressors and maximize operation of an economizer.

FIG. 1 is a schematic diagram of an air handling unit (“AHU”) 100 for conditioning air within a building or other structure (not shown). In the embodiment shown, the AHU 100 is a rooftop unit, although other AHU configurations can be used. The AHU 100 includes a temperature system 102 that includes a control unit 104 to control a condenser 108, an expansion valve 112, a direct expansion (“DX”) coil 116, and a compressor 120. The compressor 120 can generally be driven by an internal combustion engine and a standby electric motor.

In some embodiments, the AHU 100 has one or more stages of compressors. The term “compressor” used herein includes multi-stage compressors, single-stage compressors, and other types of compressors. For example, when four single-stage compressors are installed in the AHU 100, the AHU 100 has four stages: 1) one compressor is on; 2) two compressors are on; 3) three compressors are on; and 4) four compressors are on.

When the temperature system 102 is operated in a cooling process, the expansion valve 112 is adjusted to direct refrigerant from the compressor 120 through the condenser 108 to the DX coil 116. An outside air duct 124 brings in outside air through an outside air damper or valve 128. An outside air temperature sensor 126 measures or senses a temperature of the outside air near the valve 128.

When the outside air passes through the DX coil 116, the refrigerant within the DX coil 116 cools the outside air to predetermined temperature set point(s) by absorbing or removing the heat or energy in the outside air. The expansion valve 112 generally regulates the amount of refrigerant passing through the DX coil 116, thereby controlling an amount of cooling applied to the outside air. The refrigerant is then compressed by the compressor 120 and condensed at the condenser 108. The cooling process is repeated.

The AHU 100 also includes a fan 132 that draws the cooled air from the DX coil 116 through a suction chamber or fan inlet 136 to a discharge chamber or fan outlet 140. The AHU 100 also includes a differential pressure or fan head sensor 144 that measures or senses a difference between a fan inlet pressure and a fan outlet pressure (i.e., a pressure differential). A plurality of sensors, such as a temperature sensor 148, a relative humidity (“RH”) sensor 152, and a pressure sensor 154, are located downstream of the fan outlet 140 and upstream of one or more outlets 156 through which cooled air is distributed, to measure or sense a temperature, a relative humidity, and a pressure of the air downstream of the fan outlet 140, respectively.

The fan 132 conveys cooled air to various zones in the building associated with the outlets 156. After distribution to the various zones, the cooled air is collected and returned through a return air inlet 172. The AHU 100 discards a portion of the returned air through a relief air outlet 176 via a controllable valve 180. The AHU 100 then mixes the remaining returned air with outside air through a mixed air valve 184, thereby producing mixed air that has a mixed air temperature and relative humidity level. A variable frequency drive (“VFD”) 188 is coupled to the fan 132 in order to run the fan 132 at different speeds. As such, the fan 132 conveys air from the fan inlet 136 to the fan outlet 140 at a variable fan airflow rate.

The AHU 100 includes a DX controller 200 that receives a plurality of air-related conditions from various sensors, such as the outside air temperature from the outside air temperature sensor 126, the mixed air temperature from a temperature sensor 164, the relative humidity level from a relative humidity sensor 168, the fan head pressure from the differential pressure sensor 144, the pressure from the pressure sensor 154, the relative humidity level from the relative humidity sensor 152, and the temperature from the temperature sensor 148. Based on analytical or other processes such as described below, the DX controller 200 generates a plurality of control signals for use with the AHU 100. For example, the DX controller 200 can generate a fan speed control signal to drive the VFD 188. Further, the DX controller 200 can generate a plurality of valve control signals to open or close the valves 180, 184, and 128. The DX controller 200 can also generate a dynamic supply air temperature set point for use with the control unit 104, as discussed in greater detail below. In some embodiments, the DX controller 200 and the control unit 104 are separate controllers (e.g., the DX controller 200 is retrofitted in an existing system that includes the control unit 104). In other embodiments, the DX controller 200 and the control unit 104 constitute an integrated unit (e.g., functionality of the DX controller 200 is incorporated in the control unit 104).

FIGS. 2 and 3 show a side view and a front view of a fan system 300 that can be used to implement the fan 132 of FIG. 1, respectively, wherein like parts are referred to with like numerals. Two total pressure traverses 302, 303 are incorporated at the fan inlet 136 and the fan outlet 140, respectively. In some embodiments, the total pressure traverse 302 at the fan inlet 136 is linked to a negative input 304 of the differential pressure sensor 144, while the total pressure traverse 303 at the fan outlet 140 is linked to a positive input 308 of the differential pressure sensor 144. The traverses may be replaced by two tubes that are located in the respective suction and discharge chambers.

In some embodiments, a tachometer (not shown) can be used to directly measure a fan speed (“N”) of the fan 132. In other embodiments, other signals may be used to determine the fan speed. While fan power can be measured using true power meters, fan power can also be measured from signals generated by a variable frequency drive in a manner known in the art.

FIG. 4 is a block diagram of the DX controller 200 of FIG. 1. The DX controller 200 includes an interface module 202 that is configured to receive a plurality of air-related conditions and system operating conditions from sensors, such as the outside air temperature sensor 126, the temperature sensor 148, the relative humidity sensor 152, the fan head sensor 144, and the pressure sensor 154. Based on some of the sensed conditions, a flow rate module 205 determines a flow rate of the air near the fan 132, and an initialization module 207 initializes operating parameters, as described in greater detail below.

A comparator module 210 receives and compares inputs. For example, the comparator module 210 compares the outside air temperature with a maximum allowable outside air temperature. The comparator module 210 also compares the outside air temperature with a maximum allowable outside air temperature corresponding to time periods during which an economizer is disabled. Additionally, the comparator module 210 compares a mixed air temperature with a mixed air temperature set point. In such a case, the comparator module 210 determines if the mixed air temperature is greater than the mixed air temperature set point, if the mixed air temperature is less than the mixed air temperature set point, or if the mixed air temperature is equal to the mixed air temperature set point.

The DX controller 200 also includes an enable module 215 to enable or disable the compressor 132 of FIG. 1 through the control unit 104, when the mixed air temperature and the outside air temperature satisfy a plurality of comparisons, such as described below. A memory module 220 stores a plurality of parameters, such as a plurality of fan curve coefficients, a minimum fan speed, a minimum flow rate, a maximum supply air temperature, and the like. An iteration module 225 iteratively determines, in a plurality of iterations for a plurality of AHU stages, a plurality of intermediate air-related conditions and system operating conditions based on a plurality of sensed conditions. For example, the iteration module 225 can communicate with an energy module 227 to determine an enthalpy value of the supply air; with a humidity ratio module 228 to determine a humidity ratio of the mixed air; and with a cooling capacity module 229 to determine a cooling capacity of the compressor(s) 132 for each of the stages.

Once the DX controller 200 has determined the intermediate air-related conditions and system operating conditions, the comparator module 210 compares at least some of the intermediate conditions with a plurality of corresponding conditions, as detailed in FIG. 5. A rejector module 230 then rejects some of the stages based on comparisons carried out in the comparator module 210. An identifier module 235 identifies a stage among the plurality of stages based on at least one comparison of the comparator module. An updater module 240 then sets a supply air temperature set point using the supply air temperature determined for the identified stage, as described below.

The DX controller 200 also includes a fan speed module 245 to adjust a speed of the fan 132 via the variable speed drive 188 after the supply air temperature set point has been determined. The DX controller 200 also includes a valve module 250 to control the valves 128, 184, and 180 such that, for example, the makeup of mixed air can be controlled.

FIG. 5 is a flow chart illustrating a supply air temperature set point identifying process 500 carried out by the DX controller 200 of FIG. 4. At block 504, the process 500 initializes system operating conditions, such as the fan speed (“N”), a design fan speed (“N_(d)”), a minimum ratio (“ω_(min)”) between a minimum fan speed (“N_(min)”), the fan power (“w_(f)”), and the like. In the embodiment shown, the process 500 also determines a plurality of air-related conditions, such as the differential pressure, a mixed air humidity ratio (“ω_(mix)”) a design airflow rate (“Q_(d)”), and the like. The determined conditions can be determined, for example, by using sensed parameters directly, performing one or more computations using sensed parameters, etc.

At block 508, the process 500 determines airflow rate (“Q”) of the fan 132 of FIG. 2 as follows. A specific equation for determining the airflow rate is used depending on a type of fan curve associated with the fan 132. Typically, there are a number of types of fan curves, such as a steep fan curve and a flat fan curve. Fans with a steep fan curve include fans whose differential pressure or fan head increases as a result of decreasing airflow rates (“Q”) at the same fan speed (“N”). Fans with a flat fan curve include fans whose differential pressure or fan head remains generally constant when the fan airflow rate (“Q”) changes. For such fans, the fan power varies significantly when the fan airflow rate changes at the same fan speed.

For example, the process 500 can use EQN. (1) to determine the fan airflow rate (“Q”), which is measured in cubic-feet-per-minute (“CFM”), for fans with a steep fan curve. EQN. (1) is based on a measured fan head (“H”), and a ratio (“ω”) between the fan speed (“N”) that is measured in revolutions-per-minute (“RPM”) and a design fan speed (“N_(d)”) that is also measured in RPM. $\begin{matrix} {Q = {\left( \frac{{- a_{1}} - \sqrt{a_{1}^{2} - {4{a_{2}\left( {a_{0}0\frac{H}{\omega^{2}}} \right)}}}}{2a_{2}} \right)\omega}} & (1) \end{matrix}$ In EQN. (1), a₀, a₁, and a₂ are fan curve coefficients obtained from the fan curve, typically provided by manufacturers of the fan 132.

Further, the process 500 can also use EQN. (2) to determine the fan airflow rate (“Q”) for fans with a flat fan curve. EQN. (2) is based on the ratio (“ω”), and a fan power (“w_(f)”). $\begin{matrix} {Q = \frac{{{- b_{1}}\omega^{2}} - \sqrt{{b_{1}^{2}\omega^{4}} - {4b_{2}{\omega\left( {{b_{0}\omega^{3}} - w_{f}} \right)}}}}{2b_{2}\omega}} & (2) \end{matrix}$ In EQN. (2), b₀, b₁, and b₂ are fan power curve coefficients, also provided by manufacturers of the fan 132. In this way, the process 500 can determine the fan airflow rate (“Q”) using either of the above equations as appropriate.

After the DX controller 200 has determined the fan airflow rate (“Q”), the process 500 compares the outside air temperature (“T_(oa)”) with a predetermined maximum outside air temperature at which an economizer (not shown) is disabled (“T_(e,high)”) at block 512. If the process 500 determines that the outside air temperature is less than the predetermined maximum outside air temperature at which the economizer is disabled, the process 500 proceeds to compare the outside air temperature with a predetermined maximum outside air temperature allowed (“T_(oa,max)”) at block 516. In some embodiments, the predetermined maximum outside air temperature allowed is about 65° F. In other embodiments, the predetermined maximum outside air temperature allowed is adjustable. If the process 500 determines that the outside air temperature is greater than the predetermined maximum outside air temperature at which the economizer is disabled, or if the process 500 determines that the outside air temperature is greater than the predetermined maximum outside air temperature, the process 500 then proceeds to block 600, described below.

If the process 500 determines that the outside air temperature is less than the predetermined maximum outside air temperature, the process 500 proceeds to compare the mixed air humidity ratio (“ω_(mix)”) with a predetermined mixed air humidity ratio (“β₀”), which is a constant such as 0.008, at block 520. If the process 500 determines that the mixed air humidity ratio is greater than the predetermined mixed air humidity ratio, the process 500 then proceeds to block 600. If the process 500 determines that the mixed air humidity ratio is less than β₀, the process 500 proceeds to set a mixed air temperature set point (“T_(mix,set)”) to a value that is less than the supply air temperature set point (“T_(s,set)”) by an amount (“δ”), at block 524. In some embodiments, the amount is about 3° F. In other embodiments, the amount is about 2° F.

The process 500 then continues to compare a mixed air temperature (“T_(mix)”) with the mixed air temperature set point (“T_(mix,set)”) at block 528. If the process 500 determines that the mixed air temperature is less than the mixed air temperature set point at block 528, the process 500 further closes the valve 128 of FIG. 1 by a predetermined amount β₁, such as 5 percent, or reduces an amount of the valve 128 that has already been opened (“D_(oa)”) by β₁ at block 530, and repeats block 528 until the mixed air temperature is equal to the mixed air temperature set point. Otherwise, if the process 500 determines that the mixed air temperature is greater than the mixed air temperature set point at block 528, the process 500 increases D_(oa) by another predetermined amount β₂, at block 532, and repeats block 528, until the mixed air temperature is equal to the mixed air temperature set point. When the mixed air temperature equals the mixed air temperature set point, the process 500 proceeds to block 534. In some embodiments, β₁ and β₂ are the same.

At block 534, the process 500 compares the ratio (“ω”) with the predetermined minimum ratio (“ω_(min)”). If the process 500 determines that the ratio (“ω”) is less than the predetermined minimum ratio (“ω_(min)”) at block 534, the process 500 increments the supply air temperature set point by an amount (“Δ”) at block 536, and repeats block 524. However, if the process 500 determines that the ratio is greater than the predetermined minimum ratio (“ω_(min)”) at block 534, the process 500 proceeds to compare D_(oa) with a predetermined amount (“β₃”), such as 95% or 0.95, to determine if the valve 128 is fully opened (β₃=1), at block 538. It is to be appreciated that when D_(oa) is one, the valve 128 is fully opened, and when D_(oa) is zero, the valve 128 is fully closed. If the process 500 determines at block 538 that D_(oa) is less than β₃, the process 500 decrements the supply air temperature set point by the amount (“Δ”) at block 540, and repeats block 524. However, if the process 500 determines at block 538 that D_(oa) is greater than β₃, the process 500 proceeds to block 600.

At block 600, the process 500 enables the compressor 120 of FIG. 1 using either mixed air conditions or outside air conditions, and proceeds to determine an initial set of conditions, including air-related conditions and system operating conditions. In some embodiments, the air-related conditions include an initial value of the supply air temperature set point (“T_(set,i) ⁰”) for the i-th compressor stage, air density, a mixed air temperature, air specific heat, a mixed air enthalpy, a mixed air humidity ratio, a relative humidity ratio, and the like. In some embodiments, the system operating conditions include a cooling capacity of the compressor, a fan speed, a fan power, and the like.

In some embodiments, the process 500 can use EQN. (3) to determine T_(set,i) ⁰ as follows. $\begin{matrix} {T_{{set},i}^{0} = \frac{T_{r} - {{A\left( {T_{r} - T_{\min}} \right)}\left( {h_{mix} - {{Min}\left( {0.008,\omega_{mix}} \right)}} \right)}}{1 - {{AC}_{p}\left( {T_{r} - T_{mix}} \right)}}} & (3) \end{matrix}$ In EQN. (3), A is a function of a cooling capacity of the compressor at the i-th stage, E_(i), that is measured in BTU/hr, air density (“ρ”) that is measured in lb-M/ft³, and the fan airflow rate (“Q”) that is measured in CFM. An example of A is shown in EQN. (4) as follows. $\begin{matrix} {A = \frac{60\rho\quad Q}{E_{i}}} & (4) \end{matrix}$

In EQN. (3), T_(r) is a room temperature measured in ° F., T_(mix) is a mixed air temperature measured in ° F., and C_(p) is an air specific heat measured in BTU/lb-m/° F. Furthermore, in EQN. (3), h_(mix) is a mixed air enthalpy measured in BTU/lb-m which can be calculated based on the measured air temperature and relative humidity. In some embodiments, the process 500 can use EQN. (5) to determine h_(mix) as follows. h _(mix=ƒ(T) _(mix,)RH_(mix))  (5) In EQN. (3), ω_(min) is a mixed air humidity ratio which can be calculated with the measured air temperature and relative humidity ratio. In some embodiments, the process 500 can use EQN. (6) to determine ω_(mix) as follows. ω_(mix)=ƒ(T_(mix,)RH_(mix))  (6) In EQNS. (5) and (6), RH_(mix) is a mixed air relative humidity that can be sensed by the sensor 168 of FIG. 1, and functions ƒ and ƒ₁ are used to determine a mixed air enthalpy and a mixed air humidity ratio in a manner known in the art. In this way, the initial conditions for all stages of the compressor 120 are determined.

Once the process 500 has determined the initial conditions at block 600, the process 500 proceeds to determine a plurality of intermediate conditions based on the initial conditions, in an iterative fashion. In particular, the process determines the plurality of intermediate conditions with EQNS. (7)-(11) in a plurality of iterations, at block 604 as follows. $\begin{matrix} {\omega_{{set},i}^{j} = {{Min}\left( {\omega_{mix},{f_{1}\left( {T_{set}^{j},0.9} \right)}} \right)}} & (7) \\ {E_{i}^{j} = {f\left( {E_{i}^{j - 1},T_{mix},T_{set}^{j - 1},T_{oa},Q_{i}^{j - 1}} \right)}} & (8) \\ {Q_{i}^{j} = \frac{E_{i}^{j}}{60\quad{\rho\left( {h_{mix} - h_{i,{set}}^{j}} \right)}}} & (9) \\ {A = \frac{60\rho\quad Q_{i}^{j}}{E_{i}^{j}}} & (10) \\ {T_{{set},i}^{j} = \frac{T_{r} - {{A\left( {T_{r} - T_{mix}} \right)}\left( {h_{mix} - {\omega_{set}^{j - 1}h_{0}}} \right)}}{1 - {{AC}_{p}\left( {T_{r} - T_{mix}} \right)}}} & (11) \end{matrix}$ In EQNS. (7)-(11), j is an iteration number, while i is a stage number of the compressor 120, and T_(r) is a room temperature. The process 500 then checks if the iteration is done at block 608. If the process 500 has not finished all iterations at block 608, the process 500 repeats block 604 with a different iteration number.

If the process 500 has finished all iterations at block 608, the process 500 then proceeds to determine (e.g., compute or identify) a supply air temperature set point using the intermediate conditions. In particular, at block 612, the process 500 determines a minimum airflow rate (“Q_(min)”) from all the stages, and compares the airflow rate of the first stage (“Q₁”) with the minimum airflow rate. If the process 500 determines that the airflow rate of the first stage is greater than the minimum airflow rate, the process 500 proceeds to block 616. Otherwise, if the process 500 determines that the airflow rate of the first stage is less than the minimum airflow rate, the process 500 proceeds to block 620.

At block 616, when the airflow rate of the first stage is greater than the minimum airflow rate, the process 500 compares the mixed air temperature (“T_(mix)”) with a maximum supply air temperature set point (“T_(s,max)”), the mixed air humidity ratio (“ω_(mix)”) with β₀, and the airflow rate of each iteration with a percentage (“β₄”) of the design airflow rate. If the process 500 determines that the mixed air temperature (“T_(mix)”) is less than the maximum supply air temperature set point (“T_(s,max)”) the mixed air humidity ratio (“ω_(mix)”) is less than β₀, and the airflow rate of each stage is less than β₄Q_(d), the process 500 keeps the compressor 120 off at block 620, and terminates. Otherwise, if the process 500 determines that the mixed air temperature (“T_(mix)”) is greater than the maximum supply air temperature set point (“T_(s,max)”), the mixed air humidity ratio (“ω_(mix)”) is greater than β₀, or the airflow rate of each iteration is greater than β₄Q_(d), the process 500 proceeds to close the valve 128 by a threshold amount (“β₅”) at block 624. In such a case, the process 500 can modulate the outside air temperature damper or valve 128 to increase the mixed air temperature (“T_(mix)”) until the airflow rate of the first iteration (“Q₁”) reaches the minimum airflow rate (“Q_(min)”), at which time the compressor 120 can be enabled.

At block 628, the process 500 compares the airflow rate of the first iteration (“Q₁”) with the minimum airflow rate (“Q_(min)”). If the airflow rate of the first iteration is greater than the minimum airflow rate, the process 500 rejects the i-th iteration, and terminates. If the airflow rate of the i-th iteration is less than the minimum airflow rate, the process 500 repeats block 624. In such a case, the process 500 can modulate the outside air temperature damper or valve 128 to increase the mixed air temperature (“T_(mix)”) until the airflow rate of the first iteration (“Q₁”) reaches the minimum airflow rate (“Q_(min)”), at which time the compressor 120 can be enabled. In this way, the process 500 can provide a minimum amount of reheat.

At block 620, the process 500 proceeds to process the intermediate conditions of the second stage. At block 632, the process 500 compares the airflow rate of a current stage (“i”) with the minimum airflow rate (“Q_(min)”). If the process 500 determines that the airflow rate of the current stage is greater than the minimum airflow rate, the process 500 compares the mixed air humidity ratio (“ω_(mix)”) of the i-th stage with β₀, at block 636. If the process 500 determines that the mixed air humidity ratio of the i-th stage is less than β₀, the process 500 proceeds to block 640.

At block 640, the process 500 compares the supply air temperature (“T_(s)”) determined at the current stage with a temperature constant (“β₅”), such as 45° F. If the process 500 determines that the supply air temperature determined at the current stage is less than ⊖₅, the mixed air humidity ratio of the current stage is greater than β₀, or the airflow rate of the current stage is less than the minimum airflow rate, the process 500 repeats block 632 by incrementing the current stage at block 644. The process 500 then determines if the incremented stage is greater than a predetermined maximum stage number (“N_(S)”) at block 646. If the process 500 determines that the incremented stage is greater than a predetermined maximum stage number, the process 500 terminates.

If, however, the process 500 determines that the supply air temperature determined at the current stage is greater than β₅, the mixed air humidity ratio of the current stage is less than β₀, and the airflow rate of the current stage is greater than the minimum airflow rate, the process 500 proceeds to identify the current stage as an optimal stage at block 648, and assigns the supply air temperature set point determined at the current stage as the optimal supply air temperature set point for the AHU 100 at block 652. The process 500 then terminates.

Once the DX controller 200 has determined the supply air temperature set point, the control unit 104 uses the supply air temperature set point to determine a number of compressor stage to enable. In this way, a close-to-optimal or optimal number of compressor stages are enabled to provide the required temperature control.

Various features and advantages of the invention are set forth in the following claims. 

1. A method of dynamically controlling a temperature control system including a compressor and being operable in a plurality of stages, the method comprising: modulating the compressor based on an initial supply air temperature set point; determining a first plurality of air-related conditions and system operating conditions; iteratively determining, for at least some of the plurality of stages, a plurality of intermediate air-related conditions and system operating conditions based on the determined first plurality of conditions; identifying an iteration among the plurality of stages; and updating the initial supply air temperature set point with a supply air temperature set point corresponding to the identified stage.
 2. The method of claim 1, wherein the air-related conditions comprise at least one of an outside air temperature, a mixed air temperature, a mixed air relative humidity, an outlet air temperature, an outlet air humidity, an outlet air pressure, a mixed air humidity ratio, an air density, an air specific heat, a fan head, a mixed air enthalpy, and an outside air enthalpy.
 3. The method of claim 1, wherein the system operating conditions comprise at least one of a fan speed, a fan air flow rate, a fan power, and a compressor cooling capacity.
 4. The method of claim 1, wherein identifying an iteration comprises comparing at least some of the intermediate conditions with a plurality of corresponding thresholds.
 5. The method of claim 4, wherein the intermediate conditions comprise a fan flow rate, a mixed air temperature, and a mixed humidity ratio, and the thresholds comprise a minimum fan flow rate, a mixed air temperature set point, and a mixed air humidity set point, wherein the comparing comprises: determining the minimum fan flow rate from the fan flow rates for all stages; comparing the fan flow rate of a first stage with the minimum fan flow rate; comparing the mixed air temperature of a stage with the mixed air temperature set point; determining the mixed humidity ratio for a stage; comparing the mixed humidity ratio with the mixed air humidity set point; and disabling the compressor when the fan flow rate of the first stage is less than the minimum fan flow rate, when the mixed air temperature of the stage is less than the mixed air temperature set point, and when the mixed humidity ratio is less than the mixed air humidity set point.
 6. The method of claim 4, wherein the intermediate conditions comprise a fan flow rate and the thresholds comprise a minimum fan flow rate, wherein the comparing comprises: comparing the fan flow rate of a stage with the minimum fan flow rate; and rejecting the stage when the fan flow rate of the stage is less than the minimum fan flow rate.
 7. The method of claim 4, wherein the intermediate conditions comprise a supply air temperature and the thresholds comprise a minimum supply air temperature, wherein the comparing comprises: comparing the supply air temperature of a stage with the minimum supply air temperature; and rejecting the stage when the supply air temperature of the stage is less than the minimum supply air temperature.
 8. The method of claim 4, further comprising adjusting an outside air valve based on the comparing.
 9. The method of claim 8, wherein adjusting an outside air valve comprises: reducing an air volume entering the valve; and increasing the air volume entering the valve.
 10. The method of claim 1, wherein iteratively determining a plurality of intermediate conditions comprises: initializing the intermediate air-related conditions and system operating conditions; and iteratively determining a supply air temperature set point for each of the plurality of stages.
 11. The method of claim 10, further comprising: determining a supply air temperature humidity ratio for each of the plurality of stages based on the iteratively determined supply air temperature set point; determining a mixed air temperature, a supply air temperature, an outside air temperature, and an airflow rate; determining a cooling capacity based on the mixed air temperature, the supply air temperature, the outside air temperature, and the airflow rate; determining a supply air enthalpy based on the iteratively determined supply air temperature set point and the supply air temperature humidity ratio; determining a room temperature; and determining a subsequent supply air temperature set point based on the room temperature, the supply air enthalpy, and the supply air temperature humidity ratio.
 12. The method of claim 1, wherein determining the first plurality of conditions comprises sensing at least some of the conditions using at least one sensor.
 13. A controller for dynamically controlling a temperature control system including a compressor and being operable in a plurality of stages, the temperature control system being operable to cool air in a location and further including a modulator configured to modulate the compressor based on an initial supply air temperature set point and a plurality of sensors operable to sense a plurality of air-related conditions and system operating conditions, the controller comprising: an iteration module operable to iteratively determine, for at least some of the plurality of stages, a plurality of intermediate air-related conditions and system operating conditions based on the sensed conditions; an identifier module operable to identify a stage among the plurality of stages; and an updater module configured to update the initial supply air temperature set point with a supply air temperature set point corresponding to the identified stage.
 14. The controller of claim 13, further comprising a comparator module configured to compare at least some of the intermediate conditions with a plurality of corresponding thresholds, wherein the identifier module is further configured to identify the stage based on at least one comparison of the comparator module.
 15. The controller of claim 14, wherein the intermediate conditions comprise a fan flow rate and the thresholds comprise a minimum fan flow rate, wherein the comparator module is further configured to compare the fan flow rate of a stage with the minimum fan flow rate, the controller further comprising a rejector module configured to reject the stage when the fan flow rate of the stage is less than the minimum fan flow rate.
 16. The controller of claim 14, wherein the intermediate conditions comprise a supply air temperature and the thresholds comprise a minimum supply air temperature, wherein the comparator module is further configured to compare the supply air temperature of the stage with the minimum supply air temperature, the controller further comprising a rejector module configured to reject the stage when the supply air temperature of the stage is less than the minimum supply air temperature.
 17. The controller of claim 13, further comprising an initialization module configured to initialize the intermediate air-related conditions and system operating conditions.
 18. The controller of claim 13, wherein the sensed conditions comprise a mixed air temperature, a supply air temperature, an outside air temperature, and a room temperature, the controller further comprising: a humidity ratio module configured to determine a supply air temperature humidity ratio for at least some of the plurality of stages based on the iteratively determined supply air temperature set point; a flow rate module configured to determine an airflow rate; a cooling capacity module configured to determine a cooling capacity based on the mixed air temperature, the supply air temperature, the outside air temperature, and the airflow rate; and an energy module configured to determine an supply air enthalpy based on the iteratively determined supply air temperature set point and the supply air temperature humidity ratio, wherein the iteration module is further configured to determine a supply air temperature set point based on the room temperature, the supply air enthalpy, and the supply air temperature humidity ratio.
 19. The controller of claim 13, wherein the controller is a retrofitted component in the temperature control system, the temperature control system further including a main controller that is different from the controller. 